DESCRIPTION: Our main objective is to examine and arrive at a conclusion on the issue of controversy in the field of preventive Dentistry regarding the use of fluoride varnish (high fluoride concentration) to remineralize early caries. Our hypothesis, which is the issue of contention, is that since early caries is a subsurface lesion beneath a surface layer, high fluoride concentrations will rapidly remineralize the porous surface layer, causing the blockage of enamel pores and thereby reducing the ionic exchange activity of surface enamel and hindering the remineralization of the underlying subsurface lesion. This study will examine the above hypothesis in order to determine whether low or high fluoride concentration agent will be the option for remineralization of early caries. our Specific Aims are (1) to use transverse microradiography (TMR) to demonstrate the exact pattern of the effect, within the internal structure of an early caries lesion, of varying fluoride concentrations applied to remineralize the lesion, 2) to determine the lesion parameters (mineral content and lesion depth) of carious lesions that are considered to be either completely remineralizable ('reversible decay') or only-arrestible with the application of fluoride varnish, and 3) to correlate the information obtained using TMR to that obtained using QLF, in order to define the QLF parameters of a remineralizable or an arrestible lesion. This information will be useful for determination of the appropriate concentration of fluoride to be incorporated into fluoride varnish, since this product is becoming increasingly used for early caries treatment in many U.S. institutions. In this study, early caries lesions will be created on 30 extracted human premolar teeth, which will be assigned randomly to 30 human volunteers. QLF images of each lesion will be taken and analyzed. Then each lesion will be cut to produce 4 enamel blocks. Then an enamel section will be produced from each block to be used as the control to determine the pre-treatment lesion parameters (mineral level and lesion depth). The 4 blocks will be worn by their assigned volunteer in 4 randomized crossover phases. During each phase, which will last 2 months, the subject will receive either of 4 treatment modalities; (V2) high fluoride concentration varnish (5% NaF), (V1) low fluoride concentration varnish (0.2% NaF), (V0) varnish without fluoride (0% NaF) application on the lesion or (S) exposure of lesion to saliva-alone. Following treatment, an enamel section will be cut from each block and processed using TMR to quantify the post-treatment lesion parameters and to produce TMR images to visualize the effect of the treatment modalities within the internal structure of the caries lesion, in order to provide further information on the exact pattern of remineralization obtained with the various treatment modalities as per the hypothesis. The TMR data will be used to evaluate the effectiveness of the high-, low-, or no-fluoride agents or saliva-alone to enhance remineralization and inhibit further demineralization. QLF data will be matched with their corresponding TMR data to determine the QLF parameters for lesions that were either completely remineralized or only arrested.